1. Field of the Invention
The invention relates generally to a device for separating solids from solids-containing liquids and, more specifically, the invention relates to a centrifuge for separating solids from a slurry aided by a flocculant.
2. Description of Related Technology
Various types of centrifuges have been used to separate solids from liquids in industrial and municipal applications for many years. Examples of dispersions requiring solids removal are paper mill wastes and municipal sewage. Such centrifuges operate continuously or batch-wise, and may take any of various forms.
Separation is often enhanced in aqueous systems by the use of water-soluble, long chain polymers (polyelectrolytes) referred to generically herein as "flocculants." Acrylamide polymers and copolymers are examples. A flocculant assists in coagulating and removing solids from water dispersions. The efficiency and performance of the flocculant is a function of the thoroughness of mixing of the flocculant with the feed slurry to achieve contact of solid particles with the flocculant polymer chains while reducing shear destruction of flocculated agglomerates.
Continuous solid bowl centrifuges ("CSBCs") have found wide acceptance. In general, a CSBC comprises a driven, rotatable solid bowl coaxial with and surrounding a driven rotatable scroll, which typically takes the form of a helix. Other forms of centrifuges may utilize a perforate basket or other element in place of a solid bowl and/or may be operated on a batch basis. The scroll contains an internal acceleration chamber which receives a solids-containing liquid (i.e, a slurry). The acceleration chamber has discharge ports communicating with a space defined between the inner bowl surface and the scroll.
A slurry feed tube delivers the solids-containing liquid to the acceleration chamber, where the slurry is accelerated toward the speed of the outer rotating bowl and discharges through the ports in the acceleration chamber into a standing wall of liquid held in place against the bowl surface by centrifugal force. If no flocculent is used, separation is effected by centrifugal force only.
Flocculants increase solid/liquid separation by molecular entanglement between the flocculants and the solids to be separated. A net-like structure is thought to assist particle coalescence and coagulation and resultant settling. Often, flocculants are charged positively or negatively to assist in particle-charge neutralization for improved coagulation and flocculation performance. Flocculants often have molecular weights in the millions or tens of millions, and solutions of flocculants may be very viscous and difficult to mix with the feed slurry.
Prior systems for delivering a flocculant solution to the slurry suffer from a number of disadvantages. Often, the flocculent solution is injected into the side of the slurry feed tube, typically upstream of the centrifuge. In one prior system, a single stream of flocculent solution is introduced to the feed slurry just as the feed slurry discharges from the slurry feed pipe into the acceleration chamber and then into the centrifuge bowl. This system typically results in a lack of effective distribution and mixing of the flocculant with the slurry solids. This is inefficient, since complete treatment and removal of fine and colloidal solid particles (called "capture") depend on flocculant molecules contacting all solids in the feed slurry. Contacting primarily large particles alone, and not treating the fine and colloidal solids, results in higher moisture content in centrifuge solids and/or incomplete capture of the fine and colloidal materials. Incomplete capture results in the presence of unwanted solids in the discharged clarified liquid (centrate).
In order to improve capture, lower flocculant concentrations may be used to increase the volume of flocculant solution to improve distribution, but this often results in partial formation of fragile flocs, which are destroyed by shear in acceleration, resulting in the presence of solids in the centrate.
In some prior systems, a flocculant is added to the slurry feed tube externally of the centrifuge, with or without the presence of static mixing elements in the feed slurry tube. In such cases, the flocculant solution is added from the side of the feed slurry, or concentrically therewith. While the improved mixing provided by the static mixer elements may improve solids/flocculant contact, this benefit may be offset due to floc shear during acceleration in the centrifuge. This approach is not always effective in improving performance, and may only be effective in older centrifuges, which provide relatively low slurry acceleration. On relatively high acceleration centrifuges this approach is generally not applicable because of increased floc shear and resultant floc destruction.
While the use of a single concentric tube for flocculant solution feed does not present significant plugging problems even when large solids are present in the slurry feed, the use of static mixing elements or other internal projections such as a flocculant feed tube extending radially into a slurry feed tube may result in serious plugging problems, and render this approach impossible. In order to avoid potential plugging, any lateral projections (i.e., any projections extending radially into or through the slurry feed tube feed line) must be avoided.